The present invention relates to a process of preparing polycarbonates and more particularly, to the process of preparing polycarbonates through a melt polymerization reaction of a dihydroxy compound with a diarylcarbonate, wherein said melt polymerization is performed in the presence of a mixed catalyst composed of an oxygen (O) or sulfur (S) containing compound having lone-pair electrons and an alkali metal or alkaline earth metal salt in an appropriate ratio at a reduced temperature by accelerating a reaction rate to obtain high quality polycarbonates with higher than 15,000 g/mole of a viscosity average molecular weight and improved color.
Polycarbonates are known to have excellent properties in terms of transparency, impact resistance, mechanical strength and heat resistance, and thus have been widely used in industry in manufacturing transparent sheets, packaging materials, vehicle bumpers, compact discs and the like.
A representative preparing method of polycarbonates is an interfacial polymerization employing aqueous bisphenol A(BPA) solution substituted with sodium and a phosgene-containing organic solution. However, there are some disadvantages regarding to safety and ecological problems in that the phosgene used as a starting material and a chlorine-containing organic solvent used in the polymerization are extremely toxic. Furthermore, it requires excessive use of water to wash off the remaining chlorine-containing salts and un-reacted reactants after the polymerization.
A melt polymerization method, which produces polycarbonates by direct polymerization of starting materials in melt state under vacuum without using any solvents, has been introduced to solve the above problems. This method has several advantages over the above-mentioned interfacial polymerization in that it is possible to lower manufacturing cost, pelletize produced polycarbonate directly without any other treatment, and perform relatively simple apparatus.
Generally, the melt polymerization is carried at a temperature of 180-350xc2x0 C. If the reaction temperature is lower than 180xc2x0 C., it is difficult to remove phenol produced as a by-product during the polymerization thus resulting in polycarbonates with low molecular weight. On the other hand, if it is higher than 350xc2x0 C., a by-product, which changes the product to a yellow color, can be generated or decomposition of the product may occur. Although it is known that the melt polymerization can be carried in the absence of a catalyst, it has to be exposed for a prolonged time at an elevated temperature of over 280xc2x0 C. to complete the reaction. So the use of an effective polymerization catalyst is critical to attain high quality polycarbonate.
Theoretically, it is required to use the same molar amounts of hydroxyl group in bisphenol A and ester group in diphenylcarbonate to obtain high molecular weight of polycarbonates. However, because diphenylcarbonate is more volatile than bisphenol A, a little more diphenylcarbonate than bisphenol-A is desirable to obtain polycarbonates with superior physical properties. Generally, the molar ratio of diphenylcarbonate/bisphenol A is within the range from 1.0 to 1.3.
There have been many prior arts disclosing the melt polymerization to prepare polycarbonates by using a catalyst (U.S. Pat. No. 5,221,761 (1993), EP Patent No. 584801 (1993), and JP Patent Publication Nos. 7-003003 (1995) and 3-174443). Previously, the melt polymerization was performed employing a catalyst used in esterification or ester exchange reaction: (i) an alkali metal or alkali earth metal oxide or hydroxide; (ii) an alkali metal or alkali earth metal organic acid of inorganic acid; (iii) a sodium or potassium salt of phenol or bisphenol A; and (iv) an organic phosphorus and the like. Besides these catalysts, a nitrogen- or phosphorus-containing basic compound has been recently used as a catalyst as shown in U.S. Pat. No. 5,142,018. However, the process of preparing polycarbonates employing the catalyst mentioned above has posed serious problems in that the reaction requires a high reaction temperature and prolonged reaction time and the product assumes a yellow color.
Consequently, development of novel catalysts is highly required in order to produce high quality polycarbonates having improved color at a reduced temperature within a short period of time.
To solve aforementioned problems of the conventional melt polymerization methods of preparing polycarbonates in the presence of the catalyst, the present invention was completed by performing the melt polymerization in the presence of a mixed catalyst composed of a oxygen (O) or sulfur (S) containing compound having lone-pair electrons and an alkali metal or alkaline earth metal salt in an appropriate ratio.
Consequently, an object of this invention is to provide a method of preparing polycarbonates by using the mixed catalyst, which expedites the rate of the polymerization at a low temperature to the production of polycarbonates with high quality and improved color.
In the process of preparing polycarbonates by melt polymerization of aromatic dihydroxy compound and diarylcarbonate in the presence of the catalyst, the present invention is characterized by using the mixed catalyst mixed in molar ratio of from 20:1 to 1:20 of an oxygen (O) or sulfur (S) containing compound having lone-pair electrons and an alkali metal or alkaline earth metal salt to produce high quality polycarbonates.
The present invention is described in detail as set forth hereunder.
The present invention relates to a process of preparing high quality polycarbonates with improved color tone by employing the mixed catalyst, which can expedite the rate of the polymerization at a low temperature.
The catalyst used in the present invention comprises a compound having lone-pair electrons of oxygen (O) or Sulfur (S) and an alkali metal or alkaline earth metal salt. The compound having lone-pair electrons in the catalyst provides excellent activity in the melt polymerization by incorporating with an alkali metal or alkaline earth metal salt. But when the compound having lone-pair electrons is used alone or when the alkali metal or alkaline earth metal salt is used alone, there is no significant catalytic activity increase in the polymerization and the polymerization rate is similar to that of the reaction performed without any catalyst. However, when the compound having lone-pair electrons is incorporated with an alkali metal or alkaline earth metal salt, it shows a synergy effect in the polymerization. In the preparation of the catalyst, it is prefer to mix to be a molar ratio of the compound having lone-pair electrons to alkali metal or alkaline earth metal salt in the range of from 20:1 to 1:20. If the ratio is below 20:1, the polymerization rate becomes slow. If it exceeds 1:20, it results in color deterioration of the resulting product.
Examples of the compound having lone-pair electrons include oxygen-containing compound, sulfur-containing compound and a mixture thereof. Particular examples of said oxygen-containing compound are tetrahydrofuran, tetrahydropyran, 1,3-dioxane, 1,4-dioxane, 1,3-dioxepane, 1,3-dioxolan-2-one, tetronic acid, trimethylene oxide, and 1,3,5-trioxane. Particular examples of said sulfur-containing compound are tetrahydrothiophene, tetrahydrothiopyran-4-one, 1,4-thioxane, 1,3-dithiane, 1,3-dithiolane, ethylene trithiocarbonate, tetrahydrothiophen-3-one, thianaphthene, trimethylene sulfide, and 1,3,5-trithiane. Besides these compounds, other oxygen-containing and sulfur-containing compounds having lone-pair electrons can also be used. Particular examples of the alkali metal or alkaline earth metal salt of the present invention include potassium acetate, sodium acetate, rubidium acetate, cesium acetate, calcium acetate, magnesium acetate, zinc acetate and the like. The more-detailed description of the present method of preparing polycarbonates by the melt polymerization of aromatic dihydroxy compound and diarylcarbonate in the presence of the mixed catalyst is given hereunder.
An aromatic dihydroxy compound, a diarylcarbonate and a catalyst are placed into a reactor and heated to 170xc2x0 C. and stayed for 1 hr to melt them. The reaction mixture is heated to 220-300xc2x0 C. and reacted for 2 hrs under atmospheric pressure and is further reacted under pressure of 170 torr for 1 hr. The pressure is decreased to below 2 torr and is then reacted for additional 2 hrs to obtain the desired product.
The molar ratio of diarylcarbonate to aromatic dihydroxy compound is preferred to be in the range of 1 to 1.3 and that of catalyst used in the reaction is in the range of 10xe2x88x927 to 10xe2x88x922, more preferably 10xe2x88x926 to 10xe2x88x923 respectively.
The aromatic dihydroxy compound of the present invention is expressed by the formula (1),
HOxe2x80x94Ar1xe2x80x94Zxe2x80x94Ar2xe2x80x94OHxe2x80x83xe2x80x83(1)
wherein Ar1 and Ar2 represent independently phenyl group or its derivatives; and Z represents a single bond or xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CON(R1)xe2x80x94 or xe2x80x94C(R2R3)xe2x80x94 linkage, where R1, R2 and R3 are independently a hydrogen atom or xe2x80x94(CH2)nCH3 respectively, where n is an integer of 0 to 4.
The diarylcarbonate of the present invention is expressed by the formula (2),
Ar3xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94Ar4xe2x80x83xe2x80x83(2)
wherein Ar3 and Ar4 are independently phenyl or its derivatives.
A viscosity average molecular weight (Mv)of the polycarbonate prepared by the above-mentioned polymerization were estimated by using the following Equation 1, based on the intrinsic viscosity ([xcex7]) measured at 25xc2x0 C. in chloroform.
[xcex7]=Kxc2x7Mvaxe2x80x83xe2x80x83Equation 1
wherein K=0.012 cm3/g and a=0.82.
And further, a color tone of the polycarbonate prepared by the above-mentioned polymerization is measured with chromoscope and a degree of the color was estimated by comparing b values.